Lattice Theories for Polymer/Small-Molecule Mixtures and the Conformational Entropy Description of the Glass Transition Temperature

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1 Lattice Theories for Polymer/Small-Molecule Mixtures and the Conformational Entropy Description of the Glass Transition Temperature Membrane osmometry and the osmotic pressure expansion. A porous membrane is inserted in the bottom of a capillary tube. The capillary leg on the left side of the membrane contains distilled water. A dilute aqueous solution of cellulase enzyme is placed on the right side of the membrane and both legs of the capillary tube are filled with liquid to the same height. The air-liquid interface in both legs of the capillary is exposed to ambient pressure p ambient, and the temperature remains constant at 37 C. Scannng electron micrographs of the membrane reveal that the pores have an average diameter of angstroms i.e.,. micron), which translates into a "molecular-weight cutoff" of 5,. Since cellulase is a biological macromolecule with an average molecular weight of 2,, the membrane prohibits the enzyme from diffusing through the pores. However, solvent molecules are allowed to pass through the pores of the membrane. Question: Does distilled water diffuse from left-to-right or from right-to-left through the membrane? Answer: Water molecules diffuse from left-to-right. Explanation. Diffusion of water molecules from left-to-right occurs through the porous membrane in response to a gradient in the chemical potential of water at the water-membrane interface on both sides of the membrane. The chemical potential of pure water on the left side is greater than that in the aqueous solution of the enzyme on the right side. The passage of water from left-to-right, across the membrane, causes the liquid level to decrease on the left side and increase on the right side of the capillary. This "capillary rise" in the right leg generates additional hydrostatic pressure that opposes the diffusion of water through the membrane. When equilibrium is established, this increase in hydrostatic pressure on the right side prevents further diffusion of the solvent water) and the flow process ceases. Alternatively, one exposes the right leg of the capillary to an increased pressure, given by p ambient + π, where π is the osmotic pressure. Now, the height of liquid in both legs of the capillary remains the same because the hydrostatic pressure gradient opposes the chemical potential gradient that was attributed initially to differences in solvent concentration on both sides of the membrane. The consequence of chemical equilibrium. When equilibrium is established, the pressure at the water-membrane interface on the left side is p Left, which is greater than ambient pressure due to the hydrostatic contribution. On the right side, the pressure at the interface between the membrane and the cellulase enzyme solution is p Right, which is

2 also greater than ambient pressure, but p Right is greater than p Left due to the osmotic pressure effect described above. The statement of chemical equilibrium for the solvent on both sides of the membrane is; µ Pure Water T, p Left ) = µ Solvent T, p Right, x Solute ) where the chemical potential of pure water on the left side of the membrane is evaluated at 37 C and pressure p Left. On the right side of the membrane, the chemical potential of water in the aqueous solution is evaluated at 37 C, pressure p Right i.e., p Right p Left = π), and cellulase enzyme mole fraction x Solute. The pressure dependence of the solvent s chemical potential at constant temperature and composition yields the partial molar volume of the solvent via a Maxwell relation based on the extensive Gibbs free energy G of a binary mixture. There are three degrees of freedom for single-phase behaviour of binary mixtures. Consequently, four independent variables are required for a complete description of an extensive thermodynamic potential, such as G. The appropriate independent variables are temperature T, pressure p, and mole numbers of the solute and solvent i.e., N Solute and N Solvent ). Hence; dg = - S dt + V dp + µ Solvent dn Solvent + µ Solute dn Solute where S and V are the extensive entropy and volume, respectively, of the binary mixture. The appropriate Maxwell relation at composition x Solute that allows one to evaluate the pressure dependence of the chemical potential is; # $ "µ Solvent "p T,N Solvent,N Solute = # $ "V "N Solvent T,p,N Solute = V Solvent The previous equation is based on the fact that second mixed partial derivatives of an exact differential, like the extensive Gibbs free energy of binary mixtures, are not affected by reversing the order in which differentiation is performed. One integrates the previous equation from pressure p Left to p Right at constant temperature and composition to obtain the effect of pressure on the chemical potential of the solvent in the aqueous solution on the right side of the membrane; 2

3 µ Solvent T,p Right,x Solute ) dµ Solvent µ Solvent T,p Left,x Solute ) p Right " = " V Solventdp p Left µ Solvent T, p Right, x Solute ) # µ Solvent T, p Left, x Solute ) $ V Solvent Pressure dependence of the partial molar volume of the solvent was neglected in the previous result. Now, the statement of chemical equilibrium for the solvent on both sides of the membrane reduces to; µ PureWater T, p Left ) = µ Solvent T, p Right, x Solute ) = µ Solvent T, p Left, x Solute )+ "V Solvent The dilute aqueous cellulase enzyme solution in the right leg of the capillary can be approximated as an ideal liquid mixture because Raoult s law is applicable at infinite dilution. Hence, the chemical potential of the solvent at temperature T, pressure p Left, and mole fraction x Solute is evaluated as follows; µ Solvent T, p Left, x Solute ) = µ Pure Water T, p Left ) + RT ln-x Solute ) In the dilute solution regime, where -x Solute is very close to unity, one expands the logarithmic term about x Solute = in the previous equation; ln-x Solute ) x Solute /2) { x Solute } 2 /3) { x Solute } 3 The statement of chemical equilibrium yields the following osmotic pressure expansion; " = µ PureWaterT, p) # µ Solvent T, p, x Solute ) V Solvent $ RT V Solvent { x Solute + x 2 2 Solute + x 3 3 Solute +... } Problem: Sketch the difference between the liquid height in both legs of the capillary as a function of cellulase enzyme concentration in dilute aqueous solutions at 37 C and 6 C. Both liquids are incompressible to a good approximation. Their densities are roughly the same and equal to that of pure water. The polymer concentration in aqueous solution C Polymer, with dimensions of grams solute per solution volume, is related to the mole fraction of solute x Solute in the dilute solution regime as follows; 3

4 x Solute = C Polymer MW Polymer C Polymer MW Polymer + V Solvent Dilute CPolymer V Solvent " Solution MW Polymer The osmotic pressure expansion is rewritten in canonical form for molecular weight and second virial coefficient determination via linear least squares analysis of π/rtc Polymer vs. C Polymer ; " RTC Polymer # $ + MW Polymer V Solvent 2 2MW Polymer $ 2 ) C + V Solvent Polymer 3 3MW Polymer ) C 2 Polymer +... The intercept yields the inverse of the polymer s molecular weight i.e., number-average molecular weight for a distribution of chain lengths) and the initial slope corresponds to the second virial coefficient B, where; B = V Solvent 2 2MW Polymer The second virial coefficient and all higher virial coefficients in the osmotic pressure expansion vanish for dilute polymer solutions at the Θ-temperature, where the chains exhibit unperturbed dimensions and π is a linear function of polymer concentration. Analogy with the virial expansion of real gases. Consider the equation of state for a van der Waals gas with attractive interaction parameter a and excluded volume parameter b that are calculated from the critical constants. In terms of molar volume v, which is equivalent to the ratio of molecular weight MW and mass density ρ, the following equations are applicable when b is significantly less than the molar volume v; p = RT v " b $ # v p 5RT = MW + " p + a $ v b # v 2 a v 2 = RT v ) = RT / + b ) v a, * + brt -. + " $ b # v b ) MW a, 2 * + brt " b 2 $ # MW 3 4

5 One invokes the following correspondences between the virial expansion of a van der Waals gas and the osmotic pressure expansion for a dilute polymer solution; i) ii) iii) iv) Gas pressure p is analogous to osmotic pressure π Gas density ρ is analogous to polymer concentration in solution C Polymer Molecular weight of the gas MW is analogous to polymer molecular weight MW Polymer The second virial coefficient vanishes and also changes sign) at the Boyle temperature, defined by T Boyle = a/br = 27/8)T Critical, where ideal behaviour is achieved for a van der Waals gas. The second virial coefficient vanishes and also changes sign) at the Θ-temperature for dilute polymer solutions, where the chains exhibit unperturbed dimensions in the absence of polymer-solvent interactions. Gas pressure scales linearly with density at T Boyle and osmotic pressure scales linearly with C Polymer at the Θ-temperature. Lattice models for athermal mixtures with excluded volume. The major objectives of this section are to introduce the concepts of lattices, multiplicity of states, and Boltzmann s entropy expression to calculate thermodynamic properties of mixtures. One of the most important results is that the second virial coefficient in the osmotic pressure expansion of a dilute polymer solution scales linearly with the excluded volume per chain molecule, identified by γ. Excluded volume is inaccessible to long chain molecules on a lattice because these sites are occupied by chains that were inserted previously on the same lattice. Each solvent molecule is modeled as a structure-less point particle that occupies one lattice site. There is no entropic contribution associated with the placement of solvent molecules in empty lattice sites. The entropy of mixing is due solely to the multiplicity of placing polymer molecules on interconnected lattice sites. Let s begin by counting the number of ways that each polymer molecule can occupy the lattice. The total volume of the lattice corresponds to solution volume V and γ represents the volume per chain that is not available to other molecules. If A, with dimensions of inverse volume, is the proportionality constant that relates the available lattice volume to the number of conformations, or multiplicity, for placing each chain on the lattice, then; Number of conformations for the st polymer chain = AV Number of conformations for the 2 nd polymer chain = A V-γ) Number of conformations for the 3 rd polymer chain = A V-2γ) Number of conformations for the i th polymer chain = A {V-[i-]γ} The multiplicity, or total number of distinguishable ways that N Polymer molecules can be inserted into the lattice is obtained via multiplication of the conformational freedom for all 5

6 chains, as summarized above, and subsequent division by N Polymer )! because all chains are identical in chemical structure. Hence, the number of distinguishable states for this binary polymer-solvent mixture is; N Polymer " = # A V $ i $ { N Polymer }! i= ) [ ] = AV ) N Polymer { N Polymer }! N Polymer # $ i $ i= ) V ) N Polymer ) * + = AV { N Polymer }! N Polymer $ # i= $ i ) V * + The corresponding entropy of the polymer solution is given by Boltzmann s expression, where S = k lnω. The multiplicity of states Ω can be evaluated from the previous expression for the solution, pure polymer, and pure solvent i.e., N Polymer = ). Explicit evaluation of the entropy of pure solvent should not be performed until one invokes the approximation that N Polymer N Polymer ; S = k ln" = kn Polymer ln AV) # k ln N Polymer! N Polymer # { }+ k + ln # i$ V In general, iγ/v is much less than unity for all values of i in the previous summation, so it is acceptable to expand the logarithmic terms and retain only the linear contribution. Since ln-x) x Ox 2 ), one obtains the following result; { }# k$ V S " kn Polymer ln AV) # k ln N Polymer! " kn Polymer ln AV) # k ln N Polymer! i= N Polymer # i= { }# k$ V 2 N Polymer N Polymer # " kn Polymer ln AV) # k ln N Polymer! { }# k$n 2 Polymer 2V i ) Next, one evaluates the extensive entropy of mixing for a binary system that contains the following number of molecules of polymer and solvent, respectively, N Polymer and N Solvent. The calculation proceeds as follows; "S mixing = S N Polymer, N Solvent ) # S N Polymer, N Solvent = ) # S N Polymer =, N Solvent ) where the last two terms in the previous equation represent pure polymer and pure solvent, respectively. As mentioned above, structureless solvent molecules do not contribute to the entropy of mixing because there is only one way to insert them in the ) * 6

7 lattice, both in the pure state and in solution. When one evaluates the entropy of the binary mixture, SN Polymer,N Solvent ) requires partial molar volumes of polymer and solvent in the expression for solution volume V i.e., V Solution ); V Solution = n Polymer V Polymer + n Solvent V Solvent where n Polymer and n Solvent represent mole numbers for polymer and solvent, respectively. When one evaluates the entropy of pure polymer in the absence of solvent, extensive volume V i.e., V Polymer ) can be calculated from the previous equation if the solvent term is omitted and the partial molar volume of the polymer is replaced by its molar volume v Polymer. Boltzmann s entropy expression yields the following entropy change upon mixing; # "S mixing = kn Avogadro n Polymer ln V Solution $ + V Polymer 2 k)n 2 Avogadro # 2 n Polymer $ * V Polymer V Solution Statistical concepts have been employed to calculate a macroscopic thermodynamic property, as illustrated above. The specific details of the lattice are no longer required, as evidenced by the fact that the lattice parameter A does not appear in the final expression for ΔS mixing. The following sequence of steps is required to relate the second virial coefficient in the osmotic pressure expansion to the excluded volume parameter γ; ) the Gibbs free energy of mixing ΔG mixing for athermal solutions is calculated from the entropy of mixing via T ΔS mixing because ΔH mixing = 2) the chemical potential difference between the solvent in its pure state and in the polymer solution is calculated from the mole number derivative of ΔG mixing at constant temperature T, pressure p, and mole numbers of polymer n Polymer i.e., via partial molar properties when the concentration dependence of the partial molar volumes of polymer and solvent are neglected) 3) there is a direct relation between osmotic pressure and the chemical potential difference between the solvent in its pure state and in the polymer solution 7

8 Replacing kn Avogadro by the gas constant and multiplying ΔS mixing by T yields an expression for the Gibbs free energy of mixing for binary polymer solutions; $ "G mixing = #RTn Polymer ln V Solution # V Polymer ) 2 RT*N n $ 2 Avogadro Polymer # V Polymer V Solution Now, one calculates the required chemical potential difference between the solvent in its pure state and in solution as follows when the partial molar volumes of polymer and solvent are independent of composition; ) $ "#G mixing "n Solvent ) T,p,n Polymer = µ Solvent T, p, x Polymer ) * µ PureSolvent T, p) $ V Solvent = *RTn Polymer )* 2 RT+N $ Avogadron Polymer V Solution 2 V Solvent 2 V Solution Division of the previous expression by the product of RT and the partial molar volume of the solvent yields a relation between excluded volume and osmotic pressure; " RT = µ PureSolventT, p) # µ Solvent T, p, x Polymer ) = n Polymer + RTV Solvent V Solution 2 $N Avogadro ) n Polymer V Solution The molar concentration of polymer in solution, given by the ratio of n Polymer and V Solution is equivalent to C Polymer, with dimensions of grams polymer per solution volume, divided by the polymer s molecular weight MW Polymer. The lattice model with excluded volume yields the following osmotic pressure expansion in which the coefficient of the term that is linear with respect to C Polymer relates the second virial coefficient and γ; " RTC Polymer = $ + MW Polymer #N Avogadro 2 2MW Polymer ) C Polymer The product of γ and N Avogadro can be interpreted as the excluded volume per mole of chain molecules, where a mole is based on the molecular weight of the entire chain, not simply the repeat unit. As mentioned from the analysis of membrane osmometry, linear 2 * ) 8

9 least squares analysis of π/rtc Polymer vs. C Polymer yields the polymer s molecular weight from the intercept and the second viral coefficient from the slope. Generalization of these results suggests that the excluded volume vanishes when the second virial coefficient is identically zero. For example, when polymer chains exhibit unperturbed dimensions in a Θ-solvent, π/rtc Polymer is not a function of the polymer concentration in solution and the excluded volume vanishes. Flory-Huggins lattice theory for flexible polymer solutions. More detailed considerations of the lattice are provided in this section for polymer-solvent mixtures that exhibit entropic and enthalpic changes upon mixing. Interconnectivity of lattice sites that are occupied by the same polymer chain imposes severe restrictions on the conformational entropy of mixing, and these constraints become more severe when two polymers are mixed, to the extent that ΔS mixing is essentially zero for polymer-polymer blends. This interconnectivity was mentioned but not addressed in much detail in the previous section. Nomenclature: The following notation is employed to construct the Flory-Huggins lattice model. N Total = total number of individual lattice sites N Solvent = number of solvent molecules, modeled as structure-less point particles. N Polymer = number of monodisperse polymer molecules, each one contains x segments xn Polymer = number of segments in the lattice that are occupied by polymer chains N Total = N Solvent + xn Polymer Assumptions. Each site in the three-dimensional lattice is large enough for either one solvent molecule or one segment of the polymer chain to reside. Hence, solvent molecules are approximately the same size as the polymer repeat unit. Segments of the polymer chain do not interact energetically with each other, but they interact nonspecifically with solvent molecules. In other words, there are no polar functional groups or charged species in the main chain or sidegroup of the polymer that interact preferentially with the solvent. Each segment of the polymer is equally accessible to the solvent. Each polymer chain contains x segments or repeat units. As illustrated in the next section, the occupational probability of each lattice site depend on the number of polymer molecules that have already been placed on the lattice. In other words, the fact that prior segments of the same chain necessarily occupy lattice sites does not affect the calculations below when subsequent segments of the chain are placed on the lattice. All of these assumptions are considered to calculate the combinatorial entropy of mixing via statistical analysis of the counting problem because each permutation is considered to be equally likely, analogous to the microcanonical ensemble in statistical thermodynamics. The 9

10 interaction free energy of mixing accounts for energetic effects between similar and dissimilar molecules but Boltzmann weighting factors are not employed to distinguish different permutations of polymer chains conformations and structureless solvent molecules on the lattice. Conformational entropy of mixing ΔS mixing. This development begins by placing i polymer molecules on the lattice and calculating the occupational probability for all x segments of the i+ st chain. For example, if there are N Total lattice sites and x interconnected sites are required for each chain molecule that has already been placed on the lattice, then there are N total ix sites available for the first segment of the next polymer molecule. Now, interconnectivity must be considered for segments 2 through x via the coordination number z, or the number of nearest neighbor sites. Typical coordination numbers are 4 for two-dimensional lattices and 6 for three-dimensional lattices. Hence, there are a maximum of z sites available for the second segment of the chain and z- sites available for segments 3 through x, but some of these sites might be occupied by polymer molecules that have already been placed on the lattice. The fraction of vacant sites on the lattice κ i after i polymer chains occupy sites is {N Total ix}/n total, and this probabilty factor κ i together with the coordination number i.e., z or z-) is required to calculate the number of sites that are available for segments 2 through x in the i+ st chain. Changes in the probability factor κ i are considered for different chains, but not for different segments of the same chain. In summary, the number of ways that each segment of the i+ st chain can be arranged on the lattice is; st segment: N Total ix 2 nd segment: zκ i 3 rd segment: z-)κ i same for all remaining segments Since all N Polymer chains are structurally identical, the multiplicity of states, or number of distinguishable ways of placing these chains on the lattice is obtained via multiplication of the occupational freedom of all x segments in all chains, and subsequent division by N Polymer!, as follows; " = N Polymer! N Polymer $ # i= N Total $ ix)z i z $) i { } x$2 ) z $ N Total *, + N Polymer x$) N Polymer! N Polymer $ # i= N Total $ ix) x This expression for Ω is not restricted to the dilute solution regime, but it is almost impossible to evaluate the combinatorial entropy of mixing, as written, because the total number of lattice sites N Total and the number of polymer molecules N Polymer are exceedingly large. Boltzmann s equation, S = klnω, is employed to calculate the conformational

11 entropy of mixing after substantial manipulation of the multiplicity of states. following approximations are invoked at low polymer concentrations; The ) The term within the product on the far right side of the previous expression for Ω [i.e. N Total ix) x ] is written as a ratio of two factorials, which simplifies to a product of x terms that are almost identical in the dilute solution regime i.e., N Solvent >> xn Polymer ); N Total " ix) x # N Total " ix)! { N Total " xi +) }! = N Total " ix) N Total " ix ") N Total " ix " 2)... N Total " ix " x +) 2) Explicit evaluation of the product in the expression for Ω, with assistance from the dilute solution approximation in step ), reduces to; N Polymer # " i= N Total # ix)! { N Total # xi +) }! = N Total! N Total # x)! N Total # 2x)!... { N Total # N Polymer #)x}! N Total # x)! N Total # 2x)!... { N Total # N Polymer #)x}! { N Total # xn Polymer }! N = Total! { N Total # xn Polymer }! = N Total! N Solvent! 3) Stirling s approximation for the factorial when n is large; $ n!= n n 2"n exp#n) + 2n + 288n #... 2 ) ln n!* 2 ln2")+ n + 2)ln n # n * nlnn # n is employed to evaluate lnω as follows;

12 " # N Total! z $ * N Polymer!N Solvent! ) N Total N Polymer x$) ln" # N Solvent + xn Polymer )ln N Solvent + xn Polymer ) $ N Solvent + xn Polymer ) $N Polymer ln N Polymer + N Polymer $ N Solvent ln N Solvent + N Solvent +x $)N Polymer lnz $) $ x $)N Polymer ln N Solvent + xn Polymer ) Now, Boltzmann s equation yields the following expression for the combinatorial entropy of the Flory-Huggins lattice model; S k = N Solvent + N Polymer )ln N Solvent + xn Polymer ) " N Solvent ln N Solvent "x ")N Polymer { " lnz ")}" N Polymer ln N Polymer Once again, the extensive entropy of mixing is calculated by evaluating the previous equation for i) dilute polymer solutions, ii) pure polymer, and iii) pure solvent. In the spirit of maintaining an incompressible system, the size of the lattice for the sum of the precursors i.e., N Polymer molecules in the absence of solvent and N Solvent molecules without polymer) is the same as the size of the lattice required to accommodate N Solvent + N Polymer molecules in dilute solution. Hence, N Solvent and N Polymer do not change from their respective undiluted states to the solution in the expressions below. "S mixing = S N Polymer, N Solvent ) # S N Polymer, N Solvent = ) # S N Polymer =, N Solvent ) = k N Solvent + N Polymer )ln N Solvent + xn Polymer ) # kn Solvent ln N Solvent # kn Polymer ln xn Polymer ) $ N $ = #kn Solvent ln Solvent xn # kn Polymer ln Polymer N Solvent + xn Polymer ) N Solvent + xn Polymer ) 2

13 There is no combinatorial entropy associated with placing identical structure-less point particles on all of the lattice sites, to simulate pure solvent. The indistinguishability of solvent molecules reduces the multiplicity of this process to unity, but justification of this statement requires l Hopital s rule for the last term in S when N Polymer =. Coordination number z appears in the expression for the combinatorial entropy of the Flory-Huggins lattice, but the final expression for the entropy of mixing does not depend on any lattice structural parameters. This fact is reassuring because the lattice is only a crutch that allows one to simulate the mixing process. From a different viewpoint, however, the coordination number of the lattice appears in the final expressions for the entropy of mixing in Guggenheim s theory that has been applied by DiMarzio and Gibbs to predict either increases or decreases in the glass transition temperature of amorphous polymers at higher diluent volume fractions. Statistical details about the lattice, or the coordination numbers of transition metal complexes, are included in the conformational entropy description of the glass transition, based on Guggenheim s theory of mixtures as described below. The quantities in curly brackets {} in the previous expression for the Flory-Huggins entropy of mixing represent volume fractions ϕ of solvent and polymer, respectively, as defined by; " Solvent = N Solvent N Solvent + xn Polymer," Polymer = xn Polymer N Solvent + xn Polymer If the numbers of molecules of polymer and solvent, N Polymer and N Solvent, are re-expressed in terms of mole numbers and Avogadro s number, with kn Avogadro = R, and y represents mole fraction, then the previous mixture composition variables can be solved for mole fractions in terms of volume fraction as follows; y Solvent = ) ) ; y = # Polymer Polymer # Polymer + x "# Polymer ) x "# Polymer # Polymer + x "# Polymer The molar entropy of mixing Δs mixing is given by; "#S mixing ) = "#s mixing R k N Solvent + N Polymer = y Solvent ln$ Solvent + y Polymer ln$ Polymer There is one major difference between this expression for the combinatorial entropy of mixing of a dilute binary polymer solution and the ideal Δs mixing for regular solutions of lowmolecular-weight molecules. The chain-like nature of one of the components dictates the need for volume fractions in the final expression for Δs mixing, whereas volume fractions are 3

14 replaced by mole fractions for the ideal entropy of mixing of regular solutions of small molecules. The extent of randomness and chaos due to the mixing process is reduced by the chain-like nature of one or both components. Comparison of equimolar binary mixtures reveals that Δs mixing is largest for regular solutions of small molecules, intermediate for polymer solutions, and smallest for polymer-polymer blends. Interaction free energy of mixing ΔG mixing and the Flory-Huggins thermodynamic interaction parameter χ. This calculation focuses on non-specific pairwise energetic interactions between species that occupy nearest neighbor sites on the lattice. These interactions are exclusively intermolecular for the solvent, but intramolecular interactions between different segments of the same polymer chain are allowed. The following nomenclature is used to describe the various types of pairwise interactions; ε SS ε PP ε SP z zϕ Solvent zϕ Polymer interaction energy between two solvent molecules interaction energy between two polymer segments on the same chain i.e., intramolecular) or on different chains i.e., intermolecular) interaction energy between one solvent molecule and one polymer segment number of nearest neighbor sites on the two- or three-dimensional lattice number of nearest neighbor sites occupied by solvent number of nearest neighbor sites occupied by polymer segments Energetic interactions within the pure solvent ϕ Solvent = ) The lattice is occupied completely by N Solvent molecules. For each site on the lattice, excluding the boundaries, there are z nearest neighbors which contain solvent molecules that interact with a solvent molecule in the site of interest, and the energy of interaction is ε SS. Hence, the interaction energy is zε SS per site, which must be multiplied by the total number of sites that contain solvent i.e., N Solvent ). However, each pairwise interaction is counted twice by this procedure, so the cumulative interaction energy when solvent molecules occupy the entire lattice is; Pairwise interaction energy for pure solvent = /2)N Solvent zε SS Energetic interactions within the undiluted polymer ϕ Polymer = ) Now, the lattice is occupied completely by N Polymer molecules with x segments per chain. Hence, xn Polymer lattice cells are required to describe the undiluted polymer. Once again, excluding the boundaries of the lattice, there are z nearest neighbor sites which contain polymer segments that interact with the segment of interest according to ε PP. The interaction energy is zε PP per lattice site, which must be multiplied by xn Polymer sites and 4

15 divided by 2 because each pairwise segment-segment interaction has been counted twice. The total interaction energy for the undiluted polymer is; Pairwise interaction energy for pure polymer = /2)xN Polymer zε PP Energetic interactions within the polymer-solvent mixture The lattice contains both solvent and polymer with volume fractions given by ϕ Solvent and ϕ Polymer, respectively. Each calculation of a pairwise interaction energy per solvent site must be multiplied by N Solvent and each calculation per site that is occupied by a polymer segment must be multiplied by xn Polymer. Energetic interactions occur between similar molecules and dissimilar molecules. As mentioned above, interactions between similar molecules are necessarily intermolecular for the solvent, but they can be either intrachain or interchain for the polymer. Consider all lattice sites that are occupied by solvent molecules; Pairwise solvent-solvent interaction energy = /2)N Solvent zϕ Solvent ε SS Pairwise polymer-solvent interaction energy = /2)N Solvent zϕ Polymer ε SP Now, consider all lattice sites that are occupied by segments of a poymer chain; Pairwise polymer-solvent interaction energy = /2)xN Polymer zϕ Solvent ε SP Pairwise polymer-polymer interaction energy = /2)xN Polymer zϕ Polymer ε PP The extensive interaction free energy of mixing is constructed by subtracting pairwise interaction energies for pure solvent and undiluted polymer from all of the possible pairwise interaction energies that exist within the solution. One obtains the following result; ΔG mixing,interaction = /2)z { N Solvent ϕ Polymer + xn Polymer ϕ Solvent )ε SP - N Solvent - ϕ Solvent )ε SS - xn Polymer - ϕ Polymer )ε PP } Substitution for polymer and solvent volume fractions yields a rather simple expression after rearrangement that illustrates how some, but not all, solvent-solvent and both intrachain and interchain polymer segment-segment interactions are disrupted in favour of non-specific solvent/polymer-segment interactions; { )} "G mixing,int eraction = z xn SolventN Polymer # SP $ N Solvent + xn # 2 SS +# PP Polymer 5

16 The Flory-Huggins polymer-solvent thermodynamic interaction parameter χ is defined in terms of the difference between pairwise interaction energies on the right side of the previous equation; { # SS +# PP )} " = z kt # SP $ 2 Final expressions for the extensive ΔG mixing,interaction ) and molar Δg mixing,interaction, per total moles of both components) interaction free energies of mixing in binary polymer-solvent solutions, provided below, are not limited to the dilute solution regime; "G mixing,int eraction = kt#n Solvent $ Polymer "G mixing,int eraction { } = "g mixing,int eraction RT kt N Solvent + N Polymer = #y Solvent $ Polymer This is analogous to the 2-parameter van Laar model for the excess nonideal Gibbs free energy of mixing, where the van Laar quantities of interest represent a dimensionless interaction parameter and the ratio of molar volumes of both components such that the effect of composition on Δg mixing,interaction i.e., per mole of mixture) requires the mole fraction of the smaller component and the volume fraction of the larger component. For regular i.e., nonideal) binary mixtures of essentially equi-sized small molecules with nonzero interaction energies, both van Laar parameters are identical and it is necessary to replace volume fraction by mole fraction in the previous equation, which reduces to the Margules -parameter symmetric model for nonideal mixing where the Flory-Huggins thermodynamic interaction parameter is analogous to the Margules constant. If the extensive interaction free energy of mixing ΔG mixing,interaction were divided by the total number of lattice sites i.e., N Solvent +xn Polymer ) instead of the total number of molecules i.e., N Solvent +N Polymer ) in the mixture, then the effect of composition on the interaction free energy of mixing, per mole of lattice sites, requires a product of the volume fraction of each component in the binary mixture. The temperature dependence of χ governs the entropic and enthalpic contributions to the interaction free energy of mixing in the previous equation. For example, standard formalism from classical thermodynamics yields the following results for the molar entropy and enthalpy of mixing due to energetic interactions; 6

17 "s mixing,int eraction = # $"g mixing,int eraction * $T ) p,composition ) - $ T, = #Ry Solvent + Polymer. / $T 2 p,composition - $ "g "h mixing,int eraction = #T 2. mixing,int eraction / $T T - $, * = #RT 2 y Solvent + Polymer. ) 2 p,composition / $T 2 p,composition If χ varies inversely with temperature, then the interaction free energy of mixing is completely enthalpic in origin, whereas a temperature-independent value for χ implies that Δg mixing,interaction is solely due to entropic effects. The distinction between entropic and enthalpic contributions to the Flory-Huggins thermodynamic interaction parameter for polymer-solvent mixtures is discussed further in Chapter#5 on Order parameters for glasses and the concentration dependence of the glass transition temperature. Complete expression for the Gibbs free energy of mixing, partial molar properties, and the osmotic pressure expansion. Developments from the previous two sections for the conformational entropy of mixing and the interaction free energy of mixing are combined to calculate the extensive Gibbs free energy of mixing; "G mixing = "G mixing,int eraction #T"S mixing = kt{ N Solvent ln$ Solvent + N Polymer ln$ Polymer + N Solvent $ Polymer } The following logical sequence of calculations is appropriate based on the previous expression for ΔG mixing ; a) b) c) d) e) Chemical potential difference between the solvent in solution and in the pure state Osmotic pressure expansion and identification of the second virial coefficient Stability criteria based on the chemical potential or activity of the solvent Critical value of the Flory-Huggins polymer-solvent thermodynamic interaction parameter at the upper critical solution temperature i.e., UCST), above which homogeneous single-phase behaviour exists. Relation between the upper critical solution temperature and the Θ-temperature Initially, one replaces the number of molecules of polymer and solvent i.e., N Polymer and N Solvent ) by the product of Avogadro s number N Avogadro and mole numbers i.e., n Polymer and n Solvent ). Now, the partial molar Gibbs free energy of mixing of the solvent can be evaluated in a straightforward manner via partial differentiation of ΔG mixing with respect to n Solvent at constant temperature T, pressure p, and moles of polymer n Polymer. The chemical potential difference between the solvent in solution and in its pure state is; 7

18 ) # µ PureSolvent µ Solvent T, p," Polymer T, p) = $G ) mixing * = + T,p,nPolymer $n Solvent 9 RT ln" Solvent + n, Solvent $" Solvent /. " Solvent - $n 9 Solvent T,p,n Polymer + n, Polymer $" Polymer / 3, " Polymer + n Solvent. " Polymer - $n Solvent T,p,n Polymer $" Polymer $n Solvent / T,p,n Polymer 6 ) 9 8 * = RT ln #" Polymer ) +. # - x, 8 / ) 2 " Polymer + 2" Polymer * + Whereas the combinatorial entropic contribution to the previous expression is restricted to the dilute solution regime, the contribution from energetic interactions is applicable to both dilute and concentrated solutions. This limitation on the conformational entropy of mixing is modified below, so that the Gibbs free energy of mixing and the chemical potential of the solvent or diluent) can be employed in an order parameter model for polymer-diluent blends at vanishingly small diluent concentration. The osmotic pressure expansion and identification of the second virial coefficient for dilute polymer solutions is obtained via i) division of the previous equation by the partial molar volume of the solvent, ii) expansion of the logarithmic term when ϕ Polymer << [i.e., ln-ϕ) ϕ /2) ϕ 2 /3) ϕ 3 - ], iii) expression of the polymer volume fraction in terms of its mass concentration in solution C Polymer and molecular weight MW Polymer, and iv) rearrangement to arrive at a virial expansion or power series for osmotic pressure with respect to C Polymer. For example; = # RT V Solvent, - ln #$ Polymer. ) + # x " = µ PureSolventT, p) # µ Solvent T, p,$ Polymer ) = V Solvent / 2 * $ Polymer + +$ Polymer ) 2 RT V Solvent, x $ Polymer + 2 # + / 2 - * $ Polymer ) When very high-molecular-weight polymer chains exhibit unperturbed dimensions in the absence of polymer-solvent interactions at the Θ-temperature, osmotic pressure scales linearly with polymer concentration and the Flory-Huggins dimensionless interaction parameter χ approaches a value of.5 in dilute solution. Flory-Huggins entropy of mixing for concentrated polymer solutions. Let s return to the initial formulation of the multiplicity of states for placing N Polymer molecules on the lattice, where each chain contains x segments. Without invoking any dilute solution approximations;

19 " # z $ * ) N Total N Polymer x$) N Polymer! N Polymer $ + i= N Total $ ix) x The corresponding extensive entropy via Boltzmann s equation is; S k $ = ln" = x #)N Polymer ln z # )# ln N Polymer! N Total + N Polymer # ) + ln, * - i=. N Total # ix) x / The combinatorial entropy of mixing is constructed from the previous equation by adjusting the size of the lattice so that it accommodates i) {N solvent + N Polymer } molecules for the mixture, ii) N Polymer molecules in the absence of solvent, and iii) N Solvent molecules in the absence of polymer. Hence, the same number of polymer or solvent molecules is present in solution and in the respective pure states. Since the multiplicity of states Ω was based on the placement of polymer chains on the lattice, without consideration of the solvent, the previous two equations are not applicable to calculate the combinatorial entropy when the polymer is absent. As illustrated by two previous lattice examples in this chapter, stucture-less solvent molecules do not contribute to the entropy of mixing because there is only one way to insert them in the lattice, both in the pure state and in solution. This convention is adopted here. Realizing that the total number of lattice sites is either i) N Solvent + xn Polymer for the mixture, or ii) xn Polymer in the absence of solvent, one obtains the following expression for the extensive entropy of mixing; "S Mixing = S N Polymer, N Solvent ) # S N Polymer, N Solvent = ) # S N Polymer =, N Solvent ) $ z # = k3 x #)N Polymer ln N Solvent + xn ) # ln N! Polymer 23 Polymer $ #k3 x #)N Polymer ln 23 z # xn Polymer = k3 x #)N Polymer ln7 Polymer + ln, N Polymer # ) + ln, * i= ) # ln N! Polymer - + N Polymer # ) + ln, * N Polymer # * i= - i= N Solvent + x[ N Polymer # i] ) x ) x x N Polymer # i N Solvent + x[ N Polymer # i] ) x N Polymer # * x N Polymer # i) x i= The denominator of the logarithmic term in the previous equation can be simplified with assistance from Sterling s approximation for the factorial of large numbers; / / /

20 N Polymer # " x N Polymer # i) x = x N Polymer N Polymer )! i= $ N Polymer # ln " x N Polymer # i i= ) x ) = xn Polymer ln x + x ln N Polymer! 2 [ ] x = xn Polymer { ln xn Polymer ) #} ) Once again, the combinatorial entropy of mixing does not depend on coordination number z of the hypothetical lattice used to obtain thermodynamic results. The following expression for ΔS Mixing is useful in Chapter 4 when the concentration dependence of the glass transition temperature is analyzed for trace amounts of plasticizer, where the slope of T g vs. diluent concentration is most pronounced; - / "S Mixing = k/ x #)N Polymer ln$ Polymer + ln /. ) N Polymer # i= N Solvent + x[ N Polymer # i] ) x N Polymer # x N Polymer # i) x i= - N Polymer # = k/ x #)N Polymer ln$ Polymer # xn Polymer { ln xn Polymer ) #}+ x 3ln N Solvent + x N Polymer # i./ i= * , { )} For concentrated polymer solutions, Flory-Huggins analysis of the extensive combinatorial entropy of mixing can be written concisely as; ΔS Mixing = fn Polymer,N Solvent ;x) where the generalized function f is temperature-independent. Chemical stability of binary mixtures. Homogeneous binary mixtures that do not exhibit phase separation impose a few restrictions on the Gibbs free energy and its concentration dependence. One of the necessary conditions for miscibility is that the Gibbs free energy of a mixture should be less than a weighted sum of pure component Gibbs free energies. In other words, ΔG mixing must be negative. However, this condition is not sufficient to achieve a single homogeneous phase. This section focuses on the chemical stability of binary mixtures via thermodynamic analysis of the Gibbs free energy. Consider N i moles of pure component i. If each component exists as a pure single phase, then the phase rule i.e., f=c-p+2=2) suggests that extensive properties like the Gibbs free energy of pure component i, G i,pure, enjoy three degrees of freedom, where an additional degree of freedom is required because extensive functions depend on total mass of the system. If temperature T, pressure p, and mole numbers N i are chosen as 2 2

21 three independent variables for a unique description of G i,pure, then Eulers integral theorem for thermodynamic state functions that are homogeneous to the first-degree with respect to their extensive independent variables yields the following result; "G ) = N i,pure i $ G i,pure T, p, N i # "N i = N i µ i,pure T, p T,p ) where µ i,pure T,p) is the chemical potential of pure component i in its reference state. Intensive thermodynamic properties like µ i,pure, which is equivalent to the molar Gibbs free energy of pure component i, are homogeneous functions of the zeroth-order with respect to molar mass. Hence, Eulers theorem reveals that µ i,pure is not a function of N i. This is consistent with the phase rule which predicts that only two degrees of freedom are required for a unique description of µ i,pure. If an r-component homogeneous mixture contains N i moles of component i, then the phase rule i.e., f=r-+2=r+) indicates that r+2 degrees of freedom are required for a unique description of extensive properties. Eulers integral theorem focuses on the extensive independent variables, yielding the following expansion in terms of partial molar properties for the Gibbs free energy of the mixture i.e., intensive variables, like T and p, are not included in the expansion); ) = N i G mixture T, p,all _ N i r $ " #G r mixture = " N i µ i T, p,composition i= #N i ) T,p,all _ N j [ j*i ] i= 2 ) where µ i is the chemical potential of component i in the mixture. The extensive Gibbs free energy of mixing is constructed as follows; r "G mixing = G mixture # $ G i,pure = $ N i µ i T, p,composition i= r i= { ) # µ i,pure T, p) } Division by the total number of moles of all components, N Total = i r {N j }, yields the molar Gibbs free energy of mixing; r "g mixing = "G mixing = # y i µ i T, p,composition N Total i= { ) $ µ i,pure T, p) } where y i is the mole fraction of component i in the mixture. With the aid of activity coefficient correlations, the previous equation is useful to generate graphs of Δg mixing vs. mole fraction of either component in binary mixtures at constant temperature and pressure. Chemical stability analysis of these graphs is discussed below.

22 Shape of Δg mixing vs. composition in binary and multicomponent mixtures. Based on the previous equation, the Gibbs free energy of mixing, per total moles of both components in binary mixtures, is; "g mixing = # y 2 ){ µ # µ,pure }+ y 2 { µ 2 # µ 2,pure } and the instantaneous slope of Δg mixing vs. the mole fraction of component 2, y 2, at constant temperature and pressure is calculated as follows; $ " $ "µ #g mixing = { µ 2 * µ 2,pure }*{ µ * µ,pure }+ y $ "µ + y 2 2 "y 2 ) T,p "y 2 ) T,p "y 2 ) T,p The last two terms on the right side of the previous equation cancel because; y dµ + y 2 dµ 2 = via the Gibbs-Duhem equation at constant temperature and pressure. Hence; $ " #g mixing = { µ 2 * µ 2,pure }* µ * µ,pure "y 2 ) T,p { } If one introduces activities a i and activity coefficients γ i such that, at constant T and p; ) = y i " i T, p,composition) µ i # µ i,pure T ) = RT ln a i T, p,composition) a i T, p,composition then it is possible to evaluate the slope of Δg mixing vs. composition i.e., mole fraction of species 2, y 2 ) in the concentration limits because; 22

23 $ " $ #g mixing = RT ln a 2 "y 2 ) T,p ) { } =; lim{ } = lim y i * a i y i * a i a $ " lim #g mixing = ++ y 2 *"y 2 ) T,p $ " lim #g mixing =,+ y 2 *"y 2 ) T,p Hence, if Δg mixing is plotted vs. y 2, then the graph begins at pure component with Δg mixing = and an infinitely negative slope, and culminates at pure component 2 with Δg mixing = and an infinitely positive slope. Some consequences of this result are that i) Δg mixing must be negative near the concentration limits for all mixtures that achieve thermodynamic equilibrium, and ii) when mixtures separate into distinct phases, these phases can be highly concentrated in one of the components, but pure-component phases are thermodynamically disallowed. These results can be extended to multicomponent mixtures. Begin with the molar Gibbs free energy of mixing, recast in terms of activities; r "g mixing = # y i µ i T, p,composition i= { ) $ µ i,pure T, p) } = RT y i r # lna i If one envisions a multidimensional plot and focuses on the slope of Δg mixing with respect to the mole fraction of component k, then it is necessary to vary the mole fraction of one other component i.e., y r, for example). Changes in y r are not independent, but they are equal and opposite to the changes in y k to insure that all mole fractions sum to unity. The following partial derivative is of interest; $ " #g mixing "y k ) T,p,all _ y j j*k,r r + $ "y = RT ln a i - i - "y k [ ] i=, ) T,p,all _ y j j*k,r i= $ " lna + y i i "y k [ ] ) T,p,all _ y j j*k,r and the 2 nd term in the summation of the previous equation vanishes via the Gibbs-Duhem equation at constant temperature and pressure; r " y i dµ i = RT" y i d ln a i = i= r i= [ ]. / 23

24 Hence; and; $ " #g mixing "y k ) T,p,all _ y j j*k,r r $ "y = RT lna i + i "y k [ ] i= ) T,p,all _ y j [ j*k,r ] r# $ y k + y r + y j = [ ] j= j"k Since all mole fractions in the summation of the previous equation remain constant during differentiation with respect to y k, it follows that dy r = - dy k, and one obtains the following mole fraction derivatives; # ;i = k #"y $ i + + = $ *;i = r "y k T,p,all _ y j [ j)k,r ] + ;i ) k,r + The concentration dependence of Δg mixing in multicomponent mixtures is; $ " #g mixing "y k ) T,p,all _ y j j*k,r $ = RT ln a k a r [ ] ) This slope is i) infinitely positive in the limit of pure component k i.e., y k,a k, y r,a r ), and ii) infinitely negative in the limit of extremely dilute mixtures of component k i.e., y k,a k,y r >,a r >). Hence, if phase separation is inevitable and thermodynamic equilibrium is achieved, then multicomponent mixtures will not separate into pure component phases because a lower Δg mixing can be achieved if the phases are slightly impure. Intercepts and common tangents to Δg mixing vs. composition in binary mixtures. As illustrated in the previous subsection, Eulers integral theorem and the Gibbs-Duhem equation provide the tools to obtain expressions for Δg mixing, per total moles of both components, and Δg mixing / y 2 ) T,p in binary mixtures. This information allows one to evaluate the tangent line at any mixture composition via the point-slope formula. For example, if µ =µ * and µ 2 =µ 2 * when the mole fraction of component 2 is y 2 *, then; 24

25 # # "g mixing y 2 = y 2 ) = $ y 2 ){ µ # $ µ,pure }+ y # 2 { µ # 2 $ µ 2,pure } ) "g mixing * y 2 + T,p,y2 =y 2 # = { µ # 2 $ µ 2,pure }${ µ # $ µ,pure } The Taylor series expansion for the tangent line at y 2 * is truncated after the st -order term without introducing any error; " " " Tangent y 2 ; y 2 ) = #g mixing y 2 = y 2 ) + [ y 2 $ y 2 ] " = $ y 2 ){ µ " $ µ,pure }+ y " 2 µ " " { 2 $ µ 2,pure }+ y 2 $ y 2 ) µ " 2 $ µ 2,pure ) #g mixing * y 2 + " T,p,y2 =y 2 [{ }${ µ " $ µ,pure }] where Tangenty 2 ;y 2 *) represents a linear function of y 2 that is tangent to Δg mixing vs. composition at mole fraction y 2 *. Simplification of the previous equation yields; " Tangent y 2 ; y 2 ) = # y 2 ){ µ " # µ,pure }+ y 2 { µ " 2 # µ 2,pure } Evaluation of the tangent line at the pure component intercepts provides useful information about the chemical potentials of both components in the mixture at composition y 2 *. For example; ) = µ " # µ,pure ) = µ " 2 # µ 2,pure Tangent y 2 = ; y 2 " Tangent y 2 =; y 2 " Chemical stability of binary mixtures is addressed via the shape of Δg mixing vs. composition. As illustrated below, the tangent line is critical in this analysis because the pure component intercepts of a common tangent that contacts Δg mixing vs. y 2 at two different points on the curve provide the conditions for chemical equilibrium of a two-phase mixture. Homogeneous single phase behaviour occurs at all mixture compositions when both of the following conditions are satisfied; ) Δg mixing < 2) 2 Δg mixing / y 22 ) T,p > 25